Employee Benefits - Fair Value of U.S. Pension Plan Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Northern Trust Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Fair Value by Hierarchy and NAV [Extensible List]	us-gaap:FairValueMeasuredAtNetAssetValuePerShareMember	us-gaap:FairValueMeasuredAtNetAssetValuePerShareMember
Northern Trust Hedge Fund
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Fair Value by Hierarchy and NAV [Extensible List]	us-gaap:FairValueMeasuredAtNetAssetValuePerShareMember	us-gaap:FairValueMeasuredAtNetAssetValuePerShareMember
Real Estate Funds
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Fair Value by Hierarchy and NAV [Extensible List]	us-gaap:FairValueMeasuredAtNetAssetValuePerShareMember	us-gaap:FairValueMeasuredAtNetAssetValuePerShareMember
Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	$ 1,793.7	$ 1,601.2
Pension Plans, Defined Benefit | Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	13.5	12.3
Pension Plans, Defined Benefit | Domestic Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	314.4	254.6
Pension Plans, Defined Benefit | Foreign Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	43.6	45.0
Pension Plans, Defined Benefit | U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	116.0	168.3
Pension Plans, Defined Benefit | Non-U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	22.9	18.8
Pension Plans, Defined Benefit | Domestic Municipal and Provincial Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	22.4	23.1
Pension Plans, Defined Benefit | Foreign Municipal and Provincial Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.3	0.3
Pension Plans, Defined Benefit | Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	985.5	866.6
Pension Plans, Defined Benefit | Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	167.0	112.8
Pension Plans, Defined Benefit | Cash and Other
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	7.5	2.6
Pension Plans, Defined Benefit | Northern Trust Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	20.3	20.3
Pension Plans, Defined Benefit | Northern Trust Hedge Fund
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	34.2	30.2
Pension Plans, Defined Benefit | Real Estate Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	46.1	46.3
Pension Plans, Defined Benefit | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	1,693.1	1,504.4
Pension Plans, Defined Benefit | LEVEL 1
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	190.9	127.7
Pension Plans, Defined Benefit | LEVEL 1 | Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	13.5	12.3
Pension Plans, Defined Benefit | LEVEL 1 | Domestic Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 1 | Foreign Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 1 | U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	2.9	0.0
Pension Plans, Defined Benefit | LEVEL 1 | Non-U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 1 | Domestic Municipal and Provincial Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 1 | Foreign Municipal and Provincial Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 1 | Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 1 | Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	167.0	112.8
Pension Plans, Defined Benefit | LEVEL 1 | Cash and Other
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	7.5	2.6
Pension Plans, Defined Benefit | LEVEL 2
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	1,502.2	1,376.7
Pension Plans, Defined Benefit | LEVEL 2 | Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 2 | Domestic Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	314.4	254.6
Pension Plans, Defined Benefit | LEVEL 2 | Foreign Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	43.6	45.0
Pension Plans, Defined Benefit | LEVEL 2 | U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	113.1	168.3
Pension Plans, Defined Benefit | LEVEL 2 | Non-U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	22.9	18.8
Pension Plans, Defined Benefit | LEVEL 2 | Domestic Municipal and Provincial Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	22.4	23.1
Pension Plans, Defined Benefit | LEVEL 2 | Foreign Municipal and Provincial Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.3	0.3
Pension Plans, Defined Benefit | LEVEL 2 | Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	985.5	866.6
Pension Plans, Defined Benefit | LEVEL 2 | Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 2 | Cash and Other
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 3
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 3 | Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 3 | Domestic Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 3 | Foreign Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 3 | U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 3 | Non-U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 3 | Domestic Municipal and Provincial Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 3 | Foreign Municipal and Provincial Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 3 | Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 3 | Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 3 | Cash and Other
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	$ 0.0	$ 0.0